Male fruit flies don't respond well to sexual rejection. Should their attempts at courtship fail, scientists have shown that spurned male flies will actually turn to booze as a substitute for sex. Understanding why, say researchers, could allow us humans to sort out some of our own dependency issues.

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Fruit flies are very sexual beings. Put a randy male fly in close quarters with a female, and (provided the male does a halfway decent job of wooing his counterpart) the two will copulate.

Things pan out a little differently, however, if the female has already mated. If this is the case, she will refuse the male's advances at all costs — running away, kicking, and even thrusting out her ovipositor (an organ typically used for egg-laying) in protest.

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Now, a team of researchers at UC San Francisco have revealed that when given the choice, rejected male flies prefer food that has been spiked with alcohol over their normal grub. Males that had successfully mated, on the other hand, showed no preference for one meal or the other.

The team, which was led by neuroscientist Ulrike Heberlien, used this behavior to identify a signaling molecule in the flies' brains called neuropeptide F (NPF). Flies that had been denied sex were shown to have low levels of NPF; but when researchers genetically altered sex-deprived flies to produce NPF at higher levels than usual, they behaved as though they were sexually satisfied, and demonstrated no preference for alcohol-dosed food.

"Natural rewards and abused drugs affect the function of the brain's reward systems," explain Heberlein and her colleagues in the latest issue of Science. "And abnormal function of these brain regions is associated with addictive behavior."

In the case of the male fruit flies, the neural pathway that NPF uses to reinforce and reward sexual experiences has been linked to alcohol consumption.

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NPF is similar to a signaling molecule in humans called neuropeptide Y. By shedding light on the relationship between NPF and neurological reward systems linking sex and alcohol consumption, Heberlein and her colleagues think they can gain a better understanding of how addiction works in humans.

The researchers' findings are published in the latest issue of Science.Top image via